The present disclosure relates generally to automobile steering systems and, more particularly, to a method of inverter linearization in electric machines such as electric power steering (EPS) motors through secondary modulation of a motor voltage command thereto.
Electric power steering (EPS) has been recently implemented in motor vehicles to improve fuel economy and has even started to replace hydraulic power steering in certain vehicles. One way to accomplish improved fuel economy is through the reduction or elimination of losses inherent in traditional steering systems. To this end, electric power steering requires power only on demand. Commonly, in such EPS systems, an electronic controller is also configured to require significantly less power under a small or no steering input condition. This dramatic decrease from conventional steering assist is the basis of the power and fuel savings.
A polyphase permanent magnet (PM) brushless motor is typically used in EPS systems as the actuator for providing a mechanical assist to the vehicle's steering mechanism. Such a motor is generally excited with a sinusoidal field to provide lower torque ripple, noise, and vibration as compared to those motors excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. Such deviations usually result in parasitic torque ripple components at various frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.
In EPS drive systems based on a voltage mode controlled sinusoidal PM drive, a full bridge power inverter is employed to apply a pulse width modulated (PWM) voltage across the motor phases. Unfortunately, these inverters (particularly those used for sinusoidal brushless motors) suffer, however, from several linearity issues. More specifically, an inverter used in conjunction with a sinusoidal motor with phase grounding can generate considerable torque ripple at relatively low voltage commands. In addition, when the voltage command is low, the torque output versus voltage commands characteristics are fairly non linear as the result of dead time and switching time associated with the power transistors of the inverter. Accordingly, it is desirable to reduce the torque ripple and non-linearity associated with low voltage commands in EPS systems, thereby enhancing the performance of the EPS system.